This relates generally to imaging systems, and more particularly to imaging systems with analog-to-digital converter circuitry.
Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Imagers (i.e., image sensors) may be formed from a two-dimensional array of image sensing pixels. Each pixel receives incident photons (light) and converts the photons into electrical signals. Image sensors are sometimes designed to provide images to electronic devices using a Joint Photographic Experts Group (JPEG) format.
Analog pixel output signals from the image sensing pixels can be sampled using separate sample-and-hold circuitry that is coupled to the pixels. The sample-and-hold circuitry includes capacitors on which the pixel reset level and the pixel signal level are sampled and are then provided to a comparator. A ramp voltage is applied to the pixel signal level until the pixel signal level reaches the pixel reset level and the comparator output inverts. During the application of the ramp voltage, a counter is used to count how much time passes between application of the ramp voltage and the inversion of the comparator output.
Conventional ramp circuitry applies the ramp voltage to a capacitor at the input of the comparator of the sample-and-hold circuitry. However, such an arrangement requires a high pixel supply voltage in order to support a wide range of pixel output signals sampled onto the capacitor (e.g., sufficient to support the well capacity of the pixel). The capacitor is required to have a capacitance sufficient to satisfy noise requirements such as a maximum amount of thermal (k*T/C) noise, which in turn requires the ramp circuitry to have high driving capability for driving the large capacitor. Conventional capacitors used in ramp circuitry can be hundreds of femtofarads (fF). The large sample-and-hold capacitor also occupies valuable circuit area of the imager. In addition, the pixel array is typically read by scanning pixel rows in sequential order. This sequential scanning can lead to row-dependent noise in the image output signals of the pixel array. For example, transient noise in a power supply signal is consistent throughout the pixels of a row but varies between rows. It would therefore be desirable to provide imagers with improved pixel readout and analog-to-digital conversion capabilities.